Excluding the impact of the sales tax hike in April, core CPI rose 1.1 percent from a year ago, slower than the 1.3 percent increase in July. Consumer inflation remains below the 2 percent target the Bank of Japan aims to achieve by sometime next year.

"It's still a high number, but we have seen a slight deceleration since May this year. It's not a big concern but may show that the economy might not be heating up anymore, and this should be a concern for the Bank of Japan," said Takuji Okubo, principal & chief economist at Japan Macro Advisors.

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"I would say [Bank of Japan governor Kuroda] has already changed his tone. He is talking yen down and that should be helping Japan, but the reason he's doing this is obviously because he is more concerned," he said.

"Inflationary impetus may fade in the months ahead as upward pressure on prices from the Japanese yen's depreciation last year fades and output continues to languish below potential," the Asian Development Bank wrote in the update to its annual outlook on Thursday.

Among the 10 major groups in Japan's consumer price index, prices in the fuel, light and water charges group rose the most, up 6.4 percent on year.

"The CPI is likely to strengthen in the near term because the yen's recent weakening will affect import prices (energy prices in particular)," Harumi Taguchi, principal economist at IHS Global Insight wrote in a report.

"That said, the September preliminary figures suggest core prices remain somewhat weak due to slack demand while prices for fresh foods remain high. This raises concerns that higher inflationary pressure could delay the recovery in consumer spending," she said.